There are numerous applications in which current through a circuit load is to be regulated. Current regulation is used to protect circuitry from being damaged in a short circuit or overload situation. For example, the current through a power transistor of a power supply may be monitored to provide a means for limiting current flow through circuitry powered by the supply.
In its simplest form, a current sense circuit can be provided by connecting a resistor in series with a power switching transistor and monitoring the voltage drop across the resistor. The disadvantage of this monitoring circuit is that the current flow through the resistor increases the power demands placed on the supply. Minimizing power dissipation is particularly important to applications that require battery operation, such as providing power to circuitry of a cellular phone.
Power dissipation as a result of current sensing can be reduced by drawing a set fraction of the output current from the load circuitry and using this sampled current in the sensing circuitry. Briefly, the approach is to sample the current through a large device (i.e., the power transistor) with a small known amount of current through a small "pilot" device. If the two devices are transistors, the ratio of the current through the large device to the current through the pilot device may be set by fabricating the transistors such that the same ratio applies to the gate width of the large device to the gate width of the pilot device. This ratio is referred to as the scaling factor (SF). This approach is utilized in the circuits illustrated in FIGS. 1 and 2 and is described in U.S. Pat. Nos. 4,319,181, 4,553,084 and 4,820,968 to Wrathall, the inventor of the present invention.
With reference to FIG. 1, a power switching transistor 10 is connected in series with a circuit load 12 to provide a path from a first terminal (Vdd) 14 to a second terminal (electrical ground) 16 of a power source. A pilot switching transistor 18 has a source-drain conduction path that is in parallel with the source-drain conduction path of the power switching transistor. Gate electrodes 20 and 22 of the two transistors are connected to a single switch control line 24. A pilot resistor 26 is connected between electrical ground and the source of the pilot switching transistor 18. As will be explained more fully below, the pilot switching transistor and the pilot resistor provide current "mirroring" of the current through the power switching transistor 10. A current generator 28 provides a reference current (Iref) and is connected to electrical ground via a reference resistor 30. A differential amplifier 32 has inputs that are responsive to the voltage drops across the pilot and reference resistors 26 and 30.
Typically, the goal of the current sense circuit of FIG. 1 is to provide current limitation by signaling when the current drop across the pilot resistor 26 is equal to the voltage drop across the reference resistor 30. This will occur approximately at the point at which the output current (Iout) from the circuit load 12 is equal to the reference current times the scaling factor of the gate electrodes (i.e., Iout=Iref.times.SF). The resistances of the pilot and reference resistors 26 and 30 are substantially equal. In the ideal, the voltage drops across the two resistors are equal and the current through the pilot switching transistor 18 tracks precisely the current through the power switching transistor 10 in ratio of their respective gate widths.
An advantage of the prior art current sense circuit of FIG. 1 is that the use of the scaled pilot switching transistor 18 significantly reduces the power dissipation imposed by current monitoring. Power efficiency can be further enhanced by increasing the resistance of the reference resistor 30. For example, if the resistance value is doubled, the reference current can be reduced by two, which is advantageous in terms of both efficiency and control. However, a limitation of the circuit of FIG. 1 is that it exhibits a significant degree of non-linearity, and the increase in the resistance increases the non-linearity. A small pilot resistance allows the pilot circuitry to more closely resemble the operation of the power switching transistor 10. A minimal resistance places the source of the pilot switching transistor 18 at electrical ground, similar to the source of the power switching transistor. However, as the size of the pilot resistance increases, the voltage at the source of the pilot transistor 18 increases for a given current level. This voltage drop varies as a function of Iout. Consequently, as Iout increases, there is a decreasing gate-to-source potential and a decreasing drain-to-source potential for the pilot transistor. Because the two potentials vary non-proportionally with the corresponding potentials at the power switching transistor 10, the current mirroring is inaccurate.
The linearity of the current sense circuit of FIG. 1 is improved in FIG. 2 by the addition of a reference switching transistor 34 in parallel with the reference resistor 30. The gate width of the reference transistor is equal to the gate width of the pilot transistor 18. As explained fully in U.S. Pat. No. 4,820,968 to Wrathall, the addition of the reference transistor provides compensation for the non-linearities imposed as a result of the pilot resistor 26. The reference current Iref is then proportional to the current through the power switching transistor 10. When the voltages across the pilot resistor 26 and the reference resistor 30 are equal, the current ratio between Iout and Iref is then the ratio of the gate widths of the power switching transistor 10 and the pilot switching transistor 18 (i.e., SF=Iout/Iref). The addition of the reference switching transistor 34 enables the use of large resistor values without adversely affecting the current ratio.
While the circuit of FIG. 2 operates well for its intended purpose, there are concerns regarding its limitations. For circuitry that includes DMOS (double-diffused MOS) transistors, it is difficult to fabricate unit cells smaller than a unit cell of gate width. The price of accuracy is a substantial rise in current. For example, if one ampere were to flow in the circuit load 12 of FIG. 2, and the device scaling factor were 2000 (SF=2000), the reference current would be approximately 500 .mu.A. In many applications, this reference current is unacceptably high.
What is needed is a current sensing circuit that allows further reductions in the amount of reference current for given transistor fabrication techniques and parameters.